[18] Such measures can be obtained of steel for about fifteen shillings each. They are made by Mr. Chesterman of Sheffield. They can be obtained also from other makers of philosophical instruments, at prices depending upon their delicacy. Those of the greatest accuracy are somewhat costly.
[19] To avoid variations of the position in which the needle is held when marking the divisions, the edge (LL) should not be bevelled; and an upright support may be placed upon the ruler, with a ring through which the handle of the needle passes, thereby securing that the angle formed by the needle and surface of the ruler is constant, and that equal divisions are marked.
[20] Since this was printed I have observed that the above method is not identical with that described by Miss Marks, but for ordinary purposes I do not think it will be found to be inferior.
[21] For the nature and use of the vernier, a treatise on Physics or Physical Measurements may be consulted.
CHAPTER VI.
GLASS TUBING.
The diagrams given below show the sizes and thickness of the glass tubes most frequently required. In ordering, the numbers of these diagrams may be quoted, or the exact dimensions desired may be stated.